MacGinnis is educated at Stonyhurst College in England, and studies medicine at the University of Dublin. He qualifies as a house surgeon. During World War II, MacGinnis serves as surgeon in the Royal Navy.
MacGinnis plays a German sailor in the British war film 49th Parallel (1941) with Laurence Olivier, Leslie Howard, Eric Portman, and Raymond Massey. He portrays Captain MacMorris in Olivier’s version of Henry V (1944) and the title character in the film Martin Luther (1953). He plays the villainous Julian Karswell opposite American actor Dana Andrews in the British horror film Night of the Demon (1957), which is released in the United States as Curse of the Demon. He also plays Zeus opposite Honor Blackman‘s Hera in Jason and the Argonauts (1963), plays the arch-criminal A. J. Kent in an episode of Danger Man entitled “Battle of The Cameras” (1965), and has a supporting role in John Huston‘s film The Kremlin Letter (1969).